The number of digital images held by a user has been rapidly increasing with the recent widespread use of digital cameras and image scanners and the increase in capacity of recording media. In such a situation, there is an increasing need for an image search technique which allows a user to effectively view a desired image in a large amount of accumulated image groups. One of typical search methods is a method based on a similarity between a search request image (query image) held by the user and each of images in a database (see, for example, Non-patent Document 1 and Non-patent Document 2). By use of these methods, the user can acquire a desired image from the database.
Moreover, as a method for searching for an image similar to the query image, there is an image search device for calculating an inter-image distance between a query image and each set of other image data and then extracting as similar images the sets of image data in ascending order of the inter-image distance (see, for example, Patent Document 1). In Patent Document 1, a user can search for a desired image by drawing the query image even if not having the query image.    Patent Document 1: Japanese Patent Application Publication No. 2000-148795    Non-patent Document 1: M. J. Swain and D. H. Ballard, “Color indexing”, Int. J. Comput. Vision, vol. 7, no. 1, pp. 11-32, 1991.    Non-patent Document 2: Toshiaki Nakagawa, Takeshi Hara and Hiroshi Fujita, “An Image Retrieval Method Using Local Pattern Matching”, the transactions of the Institute of Electronics, Information and Communication Engineers (D-II), vol. J85-D-II, no. 1, pp. 149-152, January 2002.